Control system for automatic transmission for vehicle

ABSTRACT

When slipping is generated during the upshifting from a first gear shift stage to a second gear shift stage on ice or the like, a control for prohibiting the upshifting from the second gear shift stage to a third gear shift stage can be carried out precisely. A control system is designed so that when a slipping state is detected on ice having a lower road surface friction coefficient by a slipping state determining device, the upshifting of an automatic transmission from a second gear shift stage to a third gear shift stage is prohibited. In this control system, the time required for the upshifting from a first gear shift stage to the second gear shift stage is measured. If such time is shorter than a first predetermined time, it is determined that a slipping has been generated during the upshifting, and the upshifting from the second gear shift stage to the third gear shift stage is prohibited for a second predetermined time. Thus, even when the determination of the slipping state by the slipping-state determining device is not in time, the further upshifting of the automatic transmission is prohibited. Therefore, it is possible to reliably avoid the carrying-out of undesirable upshifting.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a control system for an automatic transmission for a vehicle, and more particularly, to a control system for an automatic transmission for a vehicle, wherein the shifting of the automatic transmission is prohibited when a slipping state of the vehicle is determined.

2. Description of the Prior Art

When a vehicle provided with a vehicle speed detecting means for detecting a vehicle speed on the basis of a rotational speed of a rotatable member of an automatic transmission, is started on ice having a small friction coefficient, the rotational speed of the rotatable member is increased if drive wheels slip. For this reason, the vehicle speed detecting means detects a higher vehicle speed, but the actual vehicle speed remains lower, because a gripping between the drive wheels and the road surface has been lost. In this case, because the vehicle speed detecting means detects the higher vehicle speed, there is a possibility that the automatic transmission is upshifted, whereby the number of rotations of the drive wheels is further increased to exert an influence to the durability of a differential or the like.

A control system is known from Japanese Patent Publication No. 64-7259, which is designed, so that when the rotational speed of the rotatable member becomes equal to or larger than a predetermined value, it is determined that the drive wheels are in a slipping state, thereby prohibiting upshifting. In the above known control system, however, even if the cancellation of the prohibition of the upshifting is not precisely carried out, there is a possibility that the prohibition of the upshifting is continued, or the prohibition of the upshifting is canceled before the elimination of the slipping state.

To solve such a problem, a control system has been proposed for an automatic transmission for a vehicle such as in Japanese Patent Application No. 8-327015, and which is designed so that it can detect with a good accuracy, that the slipping state of the drive wheels is eliminated to restore the gripping between the drive wheels and the road surface, whereby the prohibition of the shifting can be precisely canceled.

When a vehicle provided with such a control system for the automatic transmission is started on ice which has a small friction coefficient, if the slipping is generated immediately after the starting with a first gear shift stage established, an electronic control unit which has detected the slipping can provide time to carry out a control for prohibiting the upshifting from a second gear shift stage to a third gear shift stage. However, if slipping is generated during the upshifting from the first gear shift stage to the second gear shift stage, the upshifting from the second gear shift stage to the third gear shift stage is immediately started, subsequent to the completion of the upshifting from the first gear shift stage to the second gear shift stage. For this reason, there is a possibility that if the electronic control unit which has detected the slipping tries to carry out the control for prohibiting the upshifting from the second gear shift stage to the third gear shift stage, the control is not in time and hence, the undesirable upshifting from the second gear shift stage to the third gear shift stage is carried out.

SUMMARY OF THE INVENTION

The present invention has been accomplished with the above circumstance in view, and it is an object of the present invention to ensure that even when slipping is generated during the upshifting from the first gear shift stage to the second gear shift stage on the ice or the like, the control for prohibiting the upshifting from the second gear shift stage to the third gear shift stage can be carried out precisely.

To achieve the above object, there is provided a control system for an automatic transmission for a vehicle, comprising a shifting control means adapted to change the gear shift stage of the automatic transmission in accordance with the operational state of the vehicle, a vehicle speed detecting means for detecting the vehicle speed on the basis of the rotational speed of a rotatable member of the automatic transmission, a slipping-state determining means for determining the slipping state of the vehicle on the basis of the rate of variation in vehicle speed detected by the vehicle speed detecting means, and a shifting prohibiting means for prohibiting the shifting of the automatic transmission by the shifting control means, when a slipping state is determined by the slipping-state determining means. The control system further includes a shifting time measuring means for measuring the time required for the upshifting of the automatic transmission, and the shifting prohibiting means prohibits the further upshifting of the automatic transmission for a second predetermined time, when the time measured by the shifting time measuring means, is shorter than a first predetermined time.

With the above arrangement, if the slipping is generated during upshifting of the automatic transmission, the further upshifting is continuously carried out after completion of such upshifting. For this reason, there is a possibility that the shifting prohibiting control by the above-described shifting prohibiting means is not in time, and the further upshifting is carried out. However, when the time required for upshifting of the automatic transmission measured by the shifting time measuring means, is shorter than the first predetermined time, it is determined that the slipping is generated during the upshifting, and the shifting prohibiting means prohibits the further upshifting of the automatic transmission for the second predetermined time without waiting for the determination of the usual slipping state by the slipping-state determining means. Therefore, undesired upshifting can be reliably avoided.

The first predetermined time is the time which enables the generation of the slipping during the upshifting to be reliably detected. The first predetermined time is set at 320 msec in an embodiment of the present invention, but is not limited thereto. The second predetermined time is the time until the prohibition of the shifting of the automatic transmission based on the determination of the slipping state by the slipping-state determining means, is sufficiently possible. The second predetermined time is set at 500 msec in the embodiment, but is not limited thereto.

The control system further includes an acceleration calculating means for calculating the acceleration of the vehicle during traveling of the vehicle on a flat road from the throttle opening degree or the accelerator opening degree and the vehicle speed calculated by the vehicle speed detecting means for a gear shift stage at the time when the slipping state is detected by the slipping-state determining means. A lapsed-time measuring means measures the time lapsed from the time when the slipping state is determined by the slipping-state determining means, and a presumed-vehicle speed calculating means calculates a presumed vehicle speed from the amount of variation in the vehicle speed which is calculated from the vehicle acceleration calculated by the vehicle acceleration calculating means and the lapsed time measured by the lapsed-time calculating means and which corresponds to the lapsed time, and from the vehicle speed at the time when the slipping state is determined by the slipping-state determining means. A slipping-state elimination determining means determines the elimination of the slipping state by comparing the vehicle speed detected by the vehicle speed detecting means with the presumed vehicle speed calculated by the presumed vehicle speed calculating means, and a shifting-prohibition canceling means cancels the prohibition of the shifting by the shifting prohibiting means, when the elimination of the slipping state is determined by the slipping-state elimination determining means.

With the above arrangement, the means for calculating the vehicle acceleration during traveling of the vehicle on the flat road calculates the acceleration of the vehicle on the flat road from the throttle opening degree or the accelerator opening degree and the vehicle speed calculated by the vehicle speed calculating means for the gear shift stage at the time when the slipping state is determined by the slipping-state determining means, and the lapsed-time measuring means measures the time lapsed from the time point when the slipping state is detected by the slipping-state determining means. The presumed vehicle speed calculating means calculates the presumed vehicle speed from the amount of variation in vehicle speed which is calculated from the vehicle acceleration and the lapsed time and which corresponds to the lapsed time, and from the vehicle speed at the time when the slipping state is detected. The slipping state elimination determining means determines the elimination of the slipping state by comparing the vehicle speed with the presumed vehicle speed, and the shifting-prohibition canceling means cancels the prohibition of the shifting by the shifting prohibiting means, when the elimination of the slipping state is determined. Thus, when the slipping state is eliminated, the prohibition of the shifting by the shifting prohibiting means is precisely canceled. Therefore, the prohibition of the shifting is prevented from being continued after the elimination of the slipping state and from being canceled before the elimination of the slipping state.

BRIEF DESCRIPTION OF THE DRAWINGS

The mode for carrying out the present invention will now be described by way of an embodiment shown in the accompanying drawings.

FIG. 1 is an illustration of the entire arrangement of a vehicle provided with a control system for an automatic transmission for the vehicle of the present invention.

FIG. 2 is a block diagram of a control section of the present invention.

FIG. 3 is a first portion of a flow chart for a main routine thereof.

FIG. 4 is a second portion of the flow chart for the main routine thereof.

FIG. 5 is a flow chart for an on-ice determining routine of the present invention.

FIG. 6 is a flow chart for an acceleration difference calculating routine of the present invention.

FIG. 7 is a flow chart for a presumed-vehicle speed calculating routine of the present invention.

FIG. 8 is a time chart for explaining the start of the holding of a second gear shift stage due to a slipping.

FIG. 9 is a time chart for explaining the canceling of the holding of the second gear shift stage by the convergence of the slipping.

FIG. 10 is a time chart for explaining the canceling of the holding of the second gear shift stage by the full opening of a throttle.

FIG. 11 is a time chart for explaining the inhibition of the canceling of the holding of the second gear shift stage by a second-gear shift stage canceling delay timer TMYJG.

FIG. 12 is a time chart for explaining the inhibition of the canceling of the holding of the second-gear shift stage by a timer TM234K for holding the second gear shift stage before an on-ice-determination.

FIG. 13 is a time chart for explaining the inhibition of the canceling of the holding of the second gear shift stage at a lower throttle opening degree.

FIG. 14 is a time chart for explaining the operation when a shift lever is shifted from an R position to a D position.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

As shown in FIG. 1, a front engine and front drive vehicle includes left and right drive wheels W_(FL) and W_(FR) driven by an engine E through an automatic transmission T and a differential D, and left and right follower wheels W_(RL) and W_(RR) rotated with the traveling of the vehicle. An electronic control unit U receives a signal from an engine revolution-speed sensor S₁ for detecting a number of rotations per minute of a crankshaft 1 of the engine E (an engine revolution-speed Ne), a signal from an automatic transmission input rotation-speed sensor S₂ for detecting the speed of rotation of an input shaft 3 of the automatic transmission T connected to the engine E through a torque converter 2 (an automatic transmission input rotation-speed Nm), a signal from an automatic transmission output rotation-speed sensor S₃ for detecting the speed of rotation of an output shaft 4 of the automatic transmission T (an automatic transmission output rotation-speed Nc), and a signal from a throttle opening degree sensor S₄ for detecting the throttle opening degree TH which is the opening degree of a throttle valve mounted in an intake passage in the engine E. The electronic control unit U controls the gear shifting of the automatic transmission T on the basis of the signals from the engine revolution-speed sensor S₁, the automatic transmission input rotation-speed sensor S₂, the automatic transmission output rotation-speed sensor S₃ and the throttle opening degree sensor S₄. The automatic transmission output rotation-speed Nc is in a proportional relationship to the vehicle speed V and hence, the automatic transmission output rotation-speed sensor S₃ also serves as a vehicle speed sensor S₃ in this embodiment.

As shown in FIG. 2, the electronic control unit U includes a gear-shifting control means M1, a slipping-state determining means M2, a gear-shifting prohibiting means M3, a means M4 for calculating the acceleration of the vehicle during traveling on a flat road, a lapsed-time measuring means M5, a presumed-vehicle speed calculating means M6, a slipping-state canceling determining means M7, a gear-shifting prohibition canceling means M8, a gear-shifting time measuring means M9, and a torque converter slip rate detecting means M10.

The gear-shifting control means M1 controls the gear shifting of the automatic transmission T on the basis of operational state signals such as the engine revolution-speed, the absolute pressure within an intake pipe, the vehicle speed, the throttle opening degree, the gear shift stage and the like. The slipping-state determining means M2 determines the slipping state of the drive wheels W_(FL) and W_(FR) on the basis of the rate of variation in vehicle speed V detected by the vehicle speed sensor S₃. The gear-shifting prohibiting means M3 holds the gear shift stage at a second gear shift stage by prohibiting the upshifting of the automatic transmission T by the gear-shifting control means M1 for a second predetermined time (e.g., for 500 msec.), when the slipping state of the drive wheels W_(FL) and W_(FR) have been determined, or when the time required for the upshifting from a first gear shift stage to the second gear shift stage and the time determined by a gear shifting time measuring means M9 is shorter than a first predetermined time (e.g., 320 msec.).

The acceleration calculating means M4 calculates the acceleration of the vehicle on a flat road on the basis of the vehicle speed V detected by the vehicle speed sensor S₃, the throttle opening degree TH detected by the throttle opening degree sensor S₄, and a map which will be described hereinafter. The lapsed-time measuring means M5 measures the time lapsed from the determination of the slipping state by the slipping-state determining means M2. The presumed vehicle speed calculating means M6 calculates the amount of variation in vehicle speed corresponding to the lapsed time from the acceleration of the vehicle and the lapsed time, and calculates a presumed vehicle speed VYS from the amount of variation in vehicle speed and the vehicle speed at the time when the slipping state is determined.

The slipping-state canceling determining means M7 determines the canceling of the slipping state by comparing the vehicle speed V with the presumed vehicle speed VYS. The gear-shifting prohibition canceling means M8 cancels the prohibition of the gear shifting by the gear-shifting prohibiting means M3, to permit the upshifting from the second gear shift stage, when the canceling of the slipping state is determined, or when it is detected by the torque converter slip-rate detecting means M10 (which will be described hereinafter) that the slip rate e of the torque converter 2 exceeds a predetermined value (e.g., 102%).

The gear-shifting time measuring means M9 measures the time required for the upshifting from the first gear shift stage to the second gear shift stage on the basis of the automatic transmission input rotation-speed Nm detected by the automatic transmission input rotation-speed sensor S₂ and the automatic transmission output rotation-speed Nc detected by the automatic transmission output rotation-speed sensor S₃. The time required for the upshifting from the first gear shift stage to the second gear shift stage is defined as the time from the time point when a first gear shift clutch starts the canceling of the engagement to the time point when a second gear shift clutch completes the engagement, and is measured as the time until the gear rate defined by Nm/Nc is changed from the gear ratio at the first gear shift stage to the gear ratio at the second gear shift stage.

The torque converter slip-rate detecting means M10 detects the slip rate e=Nm/Ne of the torque converter 2 on the basis of the engine revolution-speed Ne detected by the engine revolution-speed sensor S₁ (i.e., the rotation speed inputted to the torque converter 2) and the automatic transmission input rotation-speed Nm (i.e., the rotation speed outputted from the torque converter 2) detected by the automatic transmission input rotation-speed sensor S₂. When a driving force is transmitted from the engine E toward the drive wheels W_(FL) and W_(FR), the slip rate e is in a range of 0%<e<100%, but when the driving force is transmitted from the drive wheels W_(FL) and W_(FR) toward the engine E, such as during engine braking, the slip rate e is in a range of e>100%.

The operation of the embodiment of the present invention having the above-described arrangement will be described in further detail with reference to flow charts in FIGS. 3 to 7.

Prior to the description, the contents of reference characters used in the flow charts will be described below.

SPKU: Acceleration difference

HPKU: On-ice determination constant

VHYJL: Lower limit vehicle speed in carrying-out holding of a second gear shift stage

VHYJH: Upper limit vehicle speed in carrying-out holding of a second gear shift stage

HVYSH: Upper limit value of presumed vehicle speed

V₀ : Vehicle speed at the start of slipping

VYS: Presumed vehicle speed

ΔVYS: Amount of variation in presumed vehicle speed

GGHH: Reference acceleration during traveling of the vehicle on a flat road at the second gear shift stage

GGFH: Acceleration ratio resulting from the division of the reference acceleration during traveling of the vehicle on a flat road at the second gear shift stage by a reference acceleration during traveling of the vehicle on the flat road at the first gear shift stage

HDELVH: Actual acceleration

TH: Throttle opening degree

TMA: On-ice determination timer

TMTH: TH return timer

TMHYJG: Delay timer for canceling the holding of the second gear shift stage

TM234K: Timer for holding the second gear shift stage before an on-ice determination

F₋₋ SLIP: Slip determining flag

F₋₋ HYJ: On-ice determining flag

F₋₋ THCL: Flag for determining the full-closing of the throttle

F₋₋ SECHLD: Flag for determining the holding of the second gear shift stage out of on-ice

F₋₋ TC: Flag for determining a slip rate of the torque converter

First, at Step S1 of a main routine shown in FIGS. 3 and 4, it is determined whether the vehicle is traveling on ice having a small road surface friction coefficient. When the vehicle is traveling on ice, the on-ice determining flag F₋₋ HYJ is set at "1", whereby the upshifting from the second gear shift stage to the third gear shift stage is inhibited. When the vehicle is not on ice, the on-ice determining flag F₋₋ HYJ is cleared to "0", whereby the upshifting is permitted. The detail of the Step S1 will be described on the basis of an on-ice determining subroutine shown in FIG. 5.

First, the case where it is determined that the vehicle is on ice to set the on-ice determining flag F₋₋ HYJ at "1" will be described below. If the gear shift stage is not the third or fourth gear shift stage at Step S31 and the on-ice determining flag F₋₋ HYJ has been cleared to "0" at Step S32, the TH return timer TMTH is set at Step S33. If the processing is shifted to Step S35 at which it is determined whether the vehicle is traveling on ice which has a small road surface friction coefficient, the vehicle speed V detected by the vehicle speed sensor S₃ is between the lower limit vehicle speed VHYJL for carrying out the holding of the second gear shift stage (e.g., 5 km/hr) and the upper limit vehicle speed VHYJH for carrying out the holding of the second gear shift stage (e.g., 120 km/hr) at subsequent Step S34.

Prior to the description referring to the flow chart, the description referring to FIG. 8 will be made. When a driver pushes down an accelerator pedal to start the vehicle, the drive wheel speed V is increased suddenly and unstably, if the drive wheels slip on the ice. At this time, when an acceleration difference SPKU which is a difference between the acceleration of the vehicle on ice and the acceleration of the vehicle during traveling of the vehicle on a usual flat road exceeds a predetermined on-ice determining constant HPKU (e.g., a variation in vehicle speed equal to 2.9 km/hr in the course of 240 msec) at a point a, the on-ice determining timer TMA is started. If the acceleration difference SPKU is larger than the on-ice determining constant HPKU during the counting operation of the on-ice determining timer TMA, i.e., if a larger acceleration is detected over a predetermined time due to the slipping of the drive wheels on ice having a smaller road surface friction coefficient, the on-ice determining flag F₋₋ HYJ is set at "1". In this manner, by use of the on-ice determining timer TMA, a momentary slipping produced when the vehicle body jumps or when the drive wheels run onto a wet iron plate, are prevented from being misjudged as slipping on ice.

This will be described with reference to the flow chart. First, at Step S35, the acceleration difference SPKU is calculated. The detail of the calculation of the acceleration difference SPKU is shown in a subroutine shown in FIG. 6. The calculation of the acceleration difference SPKU is carried out at intervals of 80 msec. First, if 80 msec has lapsed at Step S61, a reference acceleration GGHH generated during traveling of the vehicle at the second gear shift stage on a flat road, is searched at Step S62 on the basis of Gmap with the vehicle speed V and the throttle opening degree TH serving as parameters. At subsequent Step S63, an acceleration ratio GGFH resulting from the division of the reference acceleration GGHH generated during traveling of the vehicle at the second gear shift stage on the flat road by a reference acceleration generated during traveling of the vehicle at the first gear shift stage on the flat road, is searched on the basis of a ratio map with the vehicle speed V and the throttle opening degree serving as parameters. Then, at Step S64, a difference HDELVH between the this-time vehicle speed V and the last-time vehicle speed V (240 msec before) (namely, an actual acceleration) is calculated.

If the gear shift stage is not the first gear shift stage at next Step S65, i.e., if the gear shift stage is the second gear shift stage, GGHH (the reference acceleration generated during traveling of the vehicle at the second gear shift stage on the flat road) is subtracted from HDELVH (the actual acceleration at the second gear shift stage) at Step S67 to calculate an acceleration difference at the second gear shift stage. If the gear shift stage is the first gear shift stage at Step S65, HDELVH (the actual acceleration at the first gear shift stage) is multiplied at Step S66 by GGFH searched at Step S63 (the acceleration ratio determined by dividing the reference acceleration during traveling of the vehicle at the second gear shift stage on the flat road by the reference acceleration during the traveling of the vehicle at the first gear shift stage on the flat road) to calculate an actual acceleration HDELVH in the terms of a value at the second gear shift stage. At Step S67, the GGHH is subtracted from the actual acceleration HDELVH calculated in terms of the value at the second gear shift stage to calculate an acceleration difference SPKU corresponding to the second gear shift stage.

When the acceleration difference SPKU is calculated at Step S35 in the above manner, the acceleration difference SPKU is compared with the on-ice determining constant HPKU at Step S36. If the acceleration difference SPKU exceeds the on-ice determining constant HPKU, the processing is shifted to Step S37. If the slip-determining flag F₋₋ SLIP indicating that the drive wheels are slipping has been cleared to "0" at Step S37, the presumed vehicle speed VYS during the slipping is replaced by a current vehicle speed V (i.e., a vehicle speed V₀ at the time of generation of the slipping) as an initial value at Step S38, progressing to Step S41. On the other hand, if the slip determining flag F₋₋ SLIP has been already at "1" set at Step S37, a presumed vehicle speed VYS is calculated at Step S40, progressing to Step S41. The detail of the calculation of the presumed vehicle speed VYS will be described hereinafter on the basis of a subroutine shown in FIG. 7.

When the counting operation of the on-ice determining timer TMA is completed at Step S41, the slip determining flag F₋₋ SLIP is cleared to "0" at Step S42. If the gear shift stage is not a reverse gear shift stage at subsequent Step S43, it is determined that the vehicle is traveling on ice, whereby the on-ice determining flag F₋₋ HYJ is set at "1" at Step S44 and further, a torque converter slip rate determining flag F₋₋ TC which will be described hereinafter is cleared to "0" at Step S44A. If the acceleration difference SPKU is equal to or smaller than the on-ice determining constant HPKU at Step S36, the slip determining flag F₋₋ SLIP is cleared to "0" at Step S45, and the on-ice determining timer TMA is set at Step S46.

The case where it is determined that the vehicle has been moved from on the ice onto a usual road, whereby the on-ice determining flag F₋₋ HYJ is cleared to "0", will be first described with reference to FIG. 9. If the drive wheels slip with the starting on ice, the detected vehicle speed V is increased steeply, but the actual vehicle speed V(F) is increased only slightly. The actual vehicle speed V(F) during slipping of the drive wheels cannot be detected and hence, a presumed vehicle speed VYS in a case where it is supposed that the vehicle has started on a usual flat road, is calculated. When the vehicle is moved from on the ice onto the usual road surface, to reduce the slipping, and the detected vehicle speed V is reduced to become equal to or lower than the presumed vehicle speed VYS, i.e., when a relation, V≦VYS is established at a point b in FIG. 9, it is determined that the drive wheels grip the road surface, whereby the slipping is stopped, and the on-ice determining flag F₋₋ HYJ is cleared to "0".

This will be further described with reference to the flow chart. If the on-ice determining flag F₋₋ HYJ is set at "1" at Step S32 and the throttle opening degree is equal to or larger than a fully-closed throttle opening degree CTH at Step S47, the throttle return timer TMTH is set at Step S48, and a presumed vehicle speed VYS is calculated at Step S49 according to the same subroutine as at Step S40. If the presumed vehicle speed VYS is equal to or higher than the vehicle speed V at Step S50, it is determined that the slipping has been stopped, whereby the on-ice determining flag F₋₋ HYJ is cleared to "0" at Step S52, and the on-ice determining timer TMA is set at Step S53.

When the throttle opening degree becomes equal to the fully-closed throttle opening degree at Step S47, the throttle return timer TMTH set at Step S48 in the last-time loop, starts the counting. When a predetermined time (e.g., 2 sec) has lapsed at Step S51, the on-ice determining flag F₋₋ HYJ is cleared to "0" at Step S52. This is because if two seconds have lapsed after the throttle opening degree became equal to the fully-closed throttle opening degree CTH, the slipping is subsided by a reduction in driving force and hence, it is unnecessary to hold the second gear shift stage.

The calculation of the presumed vehicle speed VYS at Steps S40 and S49 will be described with reference to the flow chart in FIG. 7. First, if 240 msec has lapsed at Step S71, a reference acceleration GGHH during traveling of the vehicle at the second gear shift stage on the usual flat road is searched at Step S72 on the basis of Gmap with the presumed vehicle speed VYS and the throttle opening degree TH as parameters. If the gear shift stage is the first gear shift stage at subsequent Step S73, GGFH which is an acceleration ratio determined by dividing the reference acceleration during traveling of the vehicle at the second gear shift stage on the flat road by the reference acceleration during traveling of the vehicle at the first acceleration on the flat road, is searched on the basis of the ratio map with the presumed vehicle speed VYS and the throttle opening degree TH as parameters. If the gear shift stage is the second gear shift stage rather than the first gear shift stage at Step S73, the acceleration ratio GGFH is set at 1 at Step S75.

Then, the reference acceleration GGHH corresponding to the second gear shift and searched at Step S72 is divided by the acceleration ratio GGFH searched at Step S74 to calculate an amount ΔVYS of variation in presumed vehicle speed corresponding to the first gear shift at Step S76. In this case, if the gear shift stage is the second gear shift stage, a value calculated by multiplying the reference acceleration GGHH corresponding to the second gear shift stage by 1 set at Step S75 is defined as an amount ΔVYS of variation in presumed vehicle speed corresponding to the second gear shift stage.

If the amount ΔVYS of variation in presumed vehicle speed is positive at subsequent Step S77 and the current throttle opening degree TH is equal to or larger than 0.6/8 at Step S78, a value calculated by adding an amount ΔV of variation in presumed vehicle speed to the presumed vehicle speed VYS is defined as a new presumed vehicle speed VYS at Step S79. If the presumed vehicle speed VYS exceeds the upper limit value HVYSH (e.g., 100 km/hr) of the presumed vehicle speed at Step S80, such upper limit value HVYSH (e.g., 100 km/hr) of the presumed vehicle speed is used as the presumed vehicle speed VYS at Step S81. If the throttle opening degree TH is smaller than 0.6/8 at Step S78, the calculation of the presumed vehicle speed VYS at Steps S79 to S81 is not carried out. It should be noted that V₀ (see FIG. 8) which is a vehicle speed V at the start of slipping, namely, a vehicle speed V at a moment when the acceleration difference SPKU exceeds the on-ice determining constant HPKU, is used as an initial value of the presumed vehicle speed VYS.

In the above manner, the on-ice determination at Step S1 in the main routine shown in FIG. 3 is carried out. As a result, if the on-ice determining flag F₋₋ HYJ is set at "1" or reset at "0", the processing is advanced to Step S2. When the gear shift stage is not the reverse gear shift stage at Step S2, the processing is advanced to Step S2A, at which it is determined whether the upshifting from the first gear shift stage to sail the second gear shift stage in the current-time loop has been completed. If the upshifting in the current-time loop has been completed, the shifting time for the upshifting measured by the shifting time measuring means M9 is compared with 320 msec at Step S2B. The time required for the on-ice determination at Step S1 is 320 msec. If the shifting time is shorter than 320 msec at Step S2B, i.e., if the upshifting from the first gear shift stage to the second gear shift stage has been completed before lapsing of the time required for the on-ice determination, a delay timer TMHYJG for canceling the holding of the second gear shift stage which will be described hereinafter, is set at Step S2C.

By the fact that if the shifting time is shorter than 320 msec at Step S2B, the delay timer TMHYJG for canceling the holding of the second gear shift stage is set, as described above, the holding of the second gear shift stage at Step S16 is continued, until a counting time of 500 msec has lapsed to complete the counting operation of the delay timer TMHYJG at Step S10, and thus, the upshifting from the second gear shift stage to the third gear shift stage is prohibited to avoid the damaging of the differential D.

When the vehicle slips immediately after the starting thereof on ice, the holding of the second gear shift stage is in time, even if it is carried out after 320 msec required for the on-ice determination at Step S1. However, if the vehicle slips during the upshifting from the first gear shift stage to the second gear shift stage, the upshifting from second gear shift stage to the third gear shift stage is started immediately after completion of the upshifting from the first gear shift stage to the second gear shift stage due to a steep increase in pseudo vehicle speed V detected by the vehicle speed sensor S₄ and hence, there is a possibility that the holding of the second gear shift stage based on the on-ice determination is not in time. Thereupon, it is noticed that when the driving wheels W_(FL) and W_(FR) are in their slipping states, the load required for engagement of the second gear shift clutch of the automatic transmission T is smaller than the load on the usual road surface and for this reason, the upshifting is completed in a short time. Thus, when the shifting time required for the upshifting from the first gear shift stage to the second gear shift stage during slipping of the wheels is shorter than a predetermined time (e.g., 320 msec), it can be determined that slipping has been produced during the upshifting from the first gear shift stage to the second gear shift stage.

In this way, when slipping is produced during the upshifting from the first gear shift stage to the second gear shift stage, the holding of the second gear shift stage is forcibly continued until 500 msec has lapsed which is the counting time of the delay timer for canceling the holding of the second gear shift stage, whereby the undesirable upshifting from the second gear shift stage to the third gear shift stage can be reliably prevented.

If the vehicle is in an operational state in which the upshifting is to be carried out and the next output of a shifting command indicates the third or fourth gear shift stage at a subsequent Step S3, the processing is advanced to Step S3A. The slip rate e of the torque converter 2, detected by the torque converter slip rate detecting means M10, is compared with 102% at Step S3A. If e≧102%, the torque converter slip determining flag F₋₋ TC is set at "1" at Step S3B. If the torque converter slip determining flag F₋₋ TC has been set at "1" at Step S3C, the processing is shifted to Step S17. If the torque converter slip determining flag F₋₋ TC has been cleared to "0" at Step S3C, the processing is advanced to Step S4.

When the vehicle traveling at a high speed on the usual road surface is braked hard, to lock the drive wheels W_(FL) and W_(FR), the automatic transmission T is downshifted to the first gear shift stage. When the braking is released from this state, the speed of rotation of the drive wheels W_(FL) and W_(FR) is suddenly restored by the friction against the road surface and hence, the pseudo vehicle speed V detected by the vehicle speed sensor S₄ steeply rises from zero. If this phenomenon is misunderstood as the slipping of the drive wheels W_(FL) and W_(FR) upon the starting of the vehicle on ice, the upshifting from the second gear shift stage to the third gear shift stage is prohibited. For this reason, there is a possibility that the engine is over-revolution.

In order to discriminate the steep rising of the vehicle speed V resulting from the locking and then unlocking of the drive wheels W_(FL) and W_(RR) from the steep rising of the vehicle resulting from the slipping of the drive wheels W_(FL) and W_(RR) upon the starting of the vehicle on ice, the slip rate of the torque converter 2 is utilized. More specifically, when the drive wheels W_(FL) and W_(RR) are shifted from the locked state to the unlocked state, the driving force is transmitted from the drive wheels W_(FL) and W_(RR) toward the engine E. Therefore, when the automatic transmission input rotation-speed Nm is larger than the engine rotation-speed Ne and the slip rate e of the torque converter 2 is equal to or larger than 102% which is the predetermined value, it is determined that the drive wheels W_(FL) and W_(RR) have been shifted from the locked state to the unlocked state and thus, the processing is shifted from Step S3C to Step S17. As a result, the holding of the second gear shift stage at Step S16 is not carried out, whereby the upshifting from the second gear shift stage to the third gear shift stage is permitted to prevent the over-revolution of the engine E.

Now, when the on-ice determining flag F₋₋ HYJ has been set at "1" at Step S4 to indicate that the vehicle is traveling on ice having a smaller road surface friction coefficient, the gear shift stage is held at the second gear shift stage at Step S16 after passing through Steps S5 to S9. When the accelerator pedal is returned at that time, there is a problem in that the holding of the second gear shift stage is released, and a undesirable upshifting is carried out. This will be described below with reference to FIG. 11. When the driver continues to depress the accelerator pedal in a state in which a slipping has been produced upon the starting of the vehicle on ice, the presumed vehicle speed VYS is gradually increased to approach the vehicle speed V. For this reason, there arises a problem that when the accelerator is returned at a point c, the vehicle speed V becomes equal to lower than the presumed vehicle speed VYS at a point d, whereby the holding of the second gear shift stage is released, permitting the upshifting to be carried out. Therefore, when the throttle opening degree TH becomes a fully-closed value in a state in which the on-ice determining flag F₋₋ HYJ has been set at "1", the releasing of the holding of the second gear shift stage is inhibited, until the counting operation of the second gear shift-holding canceling delay timer TMHYJG is completed. Thus, the undesirable upshifting can be avoided.

This will be further described with reference to the flow chart. At Steps S5 to S9, when the throttle opening degree TH becomes the fully-closed value in the state in which the on-ice determining flag F₋₋ HYJ has been set at "1", the second gear shift-holding canceling delay timer TMHYJG is set. More specifically, the throttle fully-closed opening degree determining flag F₋₋ THCL is set at "1" at Step S6, when the throttle opening degree TH is not the fully-closed value at Step S5. If the throttle fully-closed opening degree determining flag F₋₋ THCL is "1" at Step S7 when the throttle opening degree TH becomes the fully-closed value at Step S5, namely, as soon as the throttle opening degree TH becomes the fully-closed value, the second gear shift-holding canceling delay timer TMHYJG is set at Step S8, and the throttle fully-closed opening degree determining flag F₋₋ THCL is cleared to "0" at Step S9.

When the on-ice determining flag F₋₋ HYJ is cleared to "0" at Step S4 due to the elimination of the slipping, the gear shift stage is held at the second gear shift stage at Step S16, until 500 msec has lapsed to complete the counting time of the second gear shift-holding canceling delay timer TMHYJG at Step S10, whereby the upshifting to the third gear shift stage is inhibited until 500 msec has lapsed.

Now, if the accelerator pedal is returned, when the on-ice determination is still not carried out at Step S4 and hence, when the on-ice determining flag F₋₋ HYJ is in the state in which it has been cleared to "0", there is a possibility that the undesired upshifting is carried out, More specifically, if the throttle opening degree is reduced at a point e in FIG. 12, after the on-ice determining timer TMA has started the counting on the basis of the fact that the acceleration difference SPKU exceeds the on-ice determining constant HPKU with the starting of the vehicle on ice and before the on-ice determining timer TMA completes the counting, the undesirable upshifting is produced at a point f. Thereupon, the counting operation of the timer TM23K for holding the second gear shift stage before the on-ice determination, is started simultaneously with the reduction in throttle opening degree TH, whereby the upshifting is inhibited until the counting of the timer TM23K is completed.

This will be further described with reference to the flow chart. If the acceleration difference SPKU is equal to or smaller than the on-ice determining constant HPKU at Step S12 to indicate the generation of no slipping, or if the return amount of the throttle opening degree is smaller (e.g., equal to or smaller than an opening degree of 3/8) at Step S13 in a state in which the on-ice determining flag F₋₋ HYJ has been cleared to "0" at Step S4; the counting time of the second gear shift-holding canceling delay timer TMHYJG is up at Step S10, and a flag F₋₋ SECHLD (which will be described hereinafter) for determining the holding of the second gear shift stage out of on the ice has been cleared to "0" at Step S11, the holding of the second gear shift stage at Step S16 is not carried out by returning of the processing via Steps S17 and S18.

However, if the acceleration difference SPKU exceeds the on-ice determining constant HPKU at Step S13 to indicate the generation of the slipping and the return amount of the throttle opening degree TH is larger at Step S14, the flag F₋₋ SECHLD for determining the holding of the second gear shift stage out of on the ice is set at "1" at Step S14 and then, the gear shift stage is held at the second gear shift stage at Step S16, until the counting of the second gear shift stage holding timer TM234K is completed at Step S15.

In the next loop, the flag F₋₋ SECHLD for determining the holding of the second gear shift stage out of on the ice has been already set at "1" at Step S11 and hence, the processing is shifted to Step S19. While the vehicle is in the state in which the acceleration difference SPKU exceeds the on-ice determining constant HPKU at Step S19 to indicate the generation of the slipping and the return amount of the throttle opening degree TH is larger at Step S20, the timer TM234K for holding the second gear shift stage before the on-ice determination, is set each time at Step S21. If the acceleration difference SPKU becomes equal to or smaller than the on-ice determining constant HPKU at Step S19, or if the return amount of the throttle opening degree TH is smaller at Step S20, the second gear stage holding timer TM234K starts the counting at Step S15. If the counting of the second gear stage holding timer TM234K is completed, the processing is shifted to Step S18, at which the second gear shift-holding determining flag F₋₋ SECHLD is cleared to "0", whereby the holding of the second gear shift stage is released.

At a throttle opening degree TH equal to or smaller than 1/8, there is no great difference between the acceleration on the ice and the acceleration on the usual road surface. For this reason, as shown in FIG. 13, a line of vehicle speed V and a line of presumed vehicle speed VYS are closer to each other, and cross each other at a point g, whereby the undesirable releasing of the holding of the second gear shift stage may be produced in some cases. Therefore, as described at Step S78 in the flow chart in FIG. 7, the calculation of the presumed vehicle speed VYS is not carried out at a throttle opening degree TH lower than 0.6/8, and the presumed vehicle speed VYS is fixed at the value V₀, which is the vehicle speed at the start of slipping. Thus, the undesired upshifting can be avoided.

The vehicle speed sensor S3 for detecting the vehicle speed V on the basis of the speed of rotation of the rotatable member of the automatic transmission T is incapable of judging the rotating direction of the rotatable member (namely, whether the vehicle is traveling forwards or in reverse). For this reason, if the shift lever is shifted from the R position to the D position in a state in which the vehicle is traveling in reverse at a high speed, there is a possibility that the third or fourth gear shift stage is established without establishment of the first gear shift stage. At the shifting of the shift lever from the R position to the D position, a third/fourth gear shift stage prohibiting timer TMRDHYJ is started as shown in FIG. 14, thereby prohibiting the upshifting to the third or fourth gear shift stage for a predetermined time (e.g., 2 sec).

In place of the throttle opening degree TH, an accelerator opening degree which is an amount of operation of the accelerator pedal, may be used.

Further, although a ratio map has been used for the calculation of the acceleration difference SPKU or the calculation of the presumed vehicle speed VYS in the flow chart in FIGS. 6 and 7, a Gmap corresponding to each of the gear shift stages may be used, and if a Gmap is used, the ratio map is not required.

If a torque transmitted to the road surface is calculated in consideration of the torque amplifying effect due to the slipping of the torque converter 2 of the automatic transmission for calculating the presumed vehicle speed VYS, and the presumed vehicle speed VYS is corrected based on the transmitted torque, a higher accuracy control can be performed.

The present embodiment is not applied to the case where a control for alleviating a shift shock (an in-gears coast control) is carried out by establishing the first gear shift stage after establishment of the third gear shift stage for a moment, when the shift lever is shifted from the N position to the D position in a state in which the engine rotation-speed is higher than an idling rotation-speed Ne. Namely, the upshifting to the third gear shift stage by the in-gears coast control is carried out as usual.

The time of 320 msec required for the on-ice determination at Step S1 has been used as the reference time for determining whether slipping has been generated during the upshifting from the first gear shift stage to the second gear shift state at Step S3B in the embodiment. However, the reference time is not necessarily set equal to the time required for the on-ice determination.

As discussed above, when the slipping is generated during the upshifting of the automatic transmission, the further upshifting is continuously carried out after the completion of such upshifting. For this reason, there is a possibility that the shifting prohibiting control by the shifting prohibiting means is not in time and hence, the further upshifting is carried out. However, when the time required for upshifting of the automatic transmission measured by the shifting-time measuring means is shorter than the first predetermined time, it is determined that the slipping has been generated during such upshifting, and the shifting prohibiting means prohibits the further upshifting of the automatic transmission for a second predetermined time without waiting for the determination of the usual slipping state by the slipping state determining means. Thus, it is possible to reliably avoid the carrying-put of the undesirable upshifting.

When the slipping state is eliminated, the prohibition of the shifting by the shifting prohibiting means is precisely cancelled. Therefore, the prohibition of the shifting is prevented from continuing after the elimination of the slipping state and from being canceled before the elimination of the slipping state.

Although the embodiment of the present invention has been described in detail, it will be understood that the present invention is not limited to the above-described embodiment, and various modifications may be made without departing from the subject matter of the invention. 

We claim:
 1. A control system for an automatic transmission for a vehicle, said automatic transmission having a rotatable member, said system comprising:a shifting control means for changing the gear shift stage of the automatic transmission in accordance with the operational state of the vehicle, a vehicle speed detecting means for detecting a vehicle speed based on the rotational speed of said rotatable member of said automatic transmission, a slipping-state determining means for determining a slipping state of the vehicle based on the rate of variation in vehicle speed detected by said vehicle speed detecting means, and a shifting prohibiting means for prohibiting the shifting of said automatic transmission by said shifting control means, when a slipping state is determined by said slipping-state determining means, a shifting time measuring means for measuring the time required for the upshifting of the automatic transmission, and wherein said shifting prohibiting means prohibits the further upshifting of said automatic transmission for a second predetermined time, when the time measured by said shifting time measuring means is shorter than a first predetermined time.
 2. A control system for an automatic transmission for a vehicle according to claim 1, further includingan acceleration calculating means for calculating the acceleration of the vehicle during traveling of the vehicle on a flat road based on the throttle opening degree and the vehicle speed calculated by said vehicle speed detecting means for a gear shift stage at the time when the slipping state is detected by said slipping-state determining means, a lapsed-time measuring means for measuring the time lapsed from the time when the slipping state is determined by said slipping-state determining means, a presumed-vehicle speed calculating means for calculating a presumed vehicle speed from the amount of variation in vehicle speed which is calculated from the vehicle acceleration calculated by said vehicle acceleration calculating means and the lapsed time measured by said lapsed-time calculating means and which corresponds to said lapsed time, and from the vehicle speed at the time when the slipping state is determined by said slipping-state determining means, a slipping-state elimination determining means for determining the elimination of the slipping state by comparing the vehicle speed detected by said vehicle speed detecting means with the presumed vehicle speed calculated by said presumed vehicle speed calculating means, and a shifting-prohibition canceling means for canceling the prohibition of the shifting by the shifting prohibiting means, when the elimination of the slipping state is determined by said slipping-state elimination determining means. 